1. Field of the Invention
The present invention relates to a polarization mode dispersion compensating apparatus (referred to as a PMD compensating apparatus hereinafter), and in particular, to a PMD compensating apparatus for compensating polarization mode dispersion of optical transmission lines which may become limitation factors for transmission rate and transmission distance in, for example, ultrahigh-speed optical communication systems.
2. Description of the Related Art
Recently, with a demand for higher transmission speeds for optical communication systems, there has been developed and practically used ultrahigh-speed optical communication systems of, for example, 40 Gbps or higher.
In optical fiber cables of such 40 Gbps or higher ultrahigh-speed optical communication systems, relatively large polarization dispersion may be caused which would be a cause of limitations on transmission rate or transmission distance. This polarization dispersion is caused as follows. Degeneracy of the base mode is resolved due to decentering of a core of an optical fiber cable 200 for transmission or due to application of non-axisymmetric stress to the core, so that a difference in propagation velocity between optical signals of two polarized wave components of TE and TM waves perpendicular to each other leads to a group delay time difference between TE and TM waves. As shown in FIG. 28, a group delay time difference is caused between TE and TM waves. As a result, the optical pulse signal spreads in the temporal axis direction, and this leads to limitation of the transmission rate and transmission distance in the communication systems.
In order to solve such problems, there is required a method of, at a receiving terminal, controlling the state of polarization and generating a group delay time difference inverse to that of the optical transmission line of the optical fiber cable 200 for transmission, thereby compensating the group delay time difference. Also, since this polarization mode dispersion of the optical transmission line changes due to change of environments, it is necessary to control the compensation amount according to environmental variations. Further, in the ultrahigh-speed optical communication systems, wavelength dependence of wavelength dispersion or polarization mode dispersion become problems, in addition to the polarization mode dispersion, and it is necessary to compensate them.
FIG. 29 is a block diagram showing a configuration of a PMD compensating apparatus equipped with a movable mirror 203 according to a prior art, as disclosed in Japanese Patent Laid-Open Publication No. 11-196046.
Referring to FIG. 29, in this PMD compensating apparatus, an inputted optical signal is split into a TM wave and a TE wave by a polarization splitter 201, and then, movement of the movable mirror 203 between positions 203a and 203b in a direction of an arrow 203c can control the delay amount of the TE wave. As a result of this, such an adjustment is achieved that the group delay time difference between the TE wave and the TM wave is substantially minimized, and then, the adjusted TM wave and the TE wave are combined again by a polarization combiner 202. Thus, the combined optical signal is outputted.
In the PMD compensating apparatus shown in FIG. 29, the inclusion of movable parts such as the movable mirror 203 causes mechanical deterioration, which leads to operational faults or the like, and this leads to such a problem that good reliability cannot be obtained. Also, this PMD compensating apparatus has another problem that it can control polarization mode dispersion but not wavelength dispersion or wavelength dependence of the polarization mode dispersion.
In order to solve the above problems, FIG. 2 of a second prior art document of the U.S. Pat. No. 6,271,952 shows a PMD compensating apparatus equipped with a differential delay system 1019 and an optical recombiner 1022 as shown in FIG. 30, and the following operation.
In order to solve the above problems, FIG. 2 of a second prior art document of the U.S. Pat. No. 6,271,952 shows a PMD compensating apparatus equipped with a differential delay system 1019 and an optical recombiner 1022 as shown in FIG. 30, and following operation is disclosed in the second prior art document.
More particularly, based on a control signal from a controller 1010, an optical signal whose polarization state is controlled by a polarization controller 1007 is split into a TM wave and a TE wave by a polarized beam splitter 1008. The split TE wave and TM wave are each outputted to the differential delay system 1019, while at the same time they are detected by detectors 1015 and 1016 and converted into electric signals, which are outputted to a dispersion measurement circuit 1017. The differential delay system 1019 applies continuous variable differential delay amount T to each of the inputted TE wave and TM wave, and outputs these two polarized waves to the optical recombiner 1022 for recombining these. The dispersion measurement circuit 1017 measures the time difference t between the split TE and TM waves, and controls the control signal to the controller 1010 so as to be the maximum based on the result of the measurement. The controller 1010 outputs a control signal 1024, which depends on the magnitude of time difference t and influences the continuous variable differential delay amount T, to the differential delay system 1019. In response to the control signal 1024, the differential delay system 1019 controls the inputted TE wave and TM wave so as to be T=t, and outputs them to the optical recombiner 1022.
In the PMD compensating apparatus of FIG. 30, the time difference t between respective optical signals split by the polarized beam splitter 1008 is measured, and by using the result of the measurement, both the polarization controller 1007 and the differential delay system 1019 are controlled, so that the control processing becomes quite difficult, and the response speed also becomes slow. Further, although polarization mode dispersion of the optical signal can be controlled, wavelength dependence of wavelength dispersion or polarization mode dispersion can not be controlled.
An essential object of the present invention is to provide a PMD compensating apparatus which solves the above-described problems and which is capable of controlling the polarization controller and the like reliably at a higher speed as compared with the prior art.
In addition to the above-mentioned object, another object of the present invention is to provide a PMD compensating apparatus which is capable of controlling the wavelength dispersion and the wavelength dependence of polarization mode dispersion.
According to one aspect of the present invention, a PMD compensating apparatus is equipped with a polarization control means, a polarization splitting and combining means, a first optical transmission line, a second optical transmission line, a first detection means, a second detection means, a first calculation means, and a first control means. The polarization control means controls a polarization state of an inputted optical signal so that a polarization axis of the optical signal becomes substantially coincident with an optical axis of an optical transmission line. The polarization splitting and combining means has first, second and third ports, and splits an optical signal outputted from the polarization control means and inputted via the first port, into optical signals of two polarized wave components substantially perpendicular to each other, and outputs the split optical signals respectively via the second and third ports. The polarization splitting and combining means further combines the two optical signals inputted via the second and third ports, and outputs a combined signal via the first port.
The first optical transmission line has a predetermined first grating, and reflects by the first grating and outputs one optical signal outputted from the second port of the polarization splitting and combining means. The second optical transmission line has a predetermined second grating, and reflects by the second grating and outputs the other optical signal outputted from the third port of the polarization splitting and combining means. The first detection means is connected to the first optical transmission line, and detects and outputs, based on an inputted optical signal outputted from the first optical transmission line, a first detection signal indicating magnitude of waveform distortion of the optical signal inputted to the first detection means. The second detection means is connected to the second optical transmission line, and detects and outputs, based on an inputted optical signal outputted from the second optical transmission line, a second detection signal indicating magnitude of waveform distortion of the optical signal inputted to the second detection means.
The first calculation means performs a predetermined calculation about the first detection signal and the second detection signal, and outputs an output signal indicating a result of the calculation. The first control means controls the polarization control means based on the output signal from the first calculation means.
According to another aspect of the present invention, a PMD compensating apparatus is equipped with a polarization control means, a polarization splitting and combining means, a first optical transmission line, a second optical transmission line and a fifth control means. The polarization control means controls a polarization state of an inputted optical signal so that a polarization axis of the optical signal becomes substantially coincident with an optical axis of an optical transmission line. The polarization splitting and combining means has first, second and third ports, splits an optical signal outputted from the polarization control means and inputted via the first port, into optical signals of two polarized wave components substantially perpendicular to each other, and outputs the split optical signals respectively via the second and third ports. The polarization splitting and combining means further combines two optical signals inputted via the second and third ports, and outputting a combined optical signal via the first port.
The first optical transmission line has a predetermined first grating, reflects by the first grating and outputs one optical signal outputted from the second port of the polarization splitting and combining means. The second optical transmission line has a predetermined second grating, and reflects by the second grating and outputs, the other optical signal outputted from the third port of the polarization splitting and combining means. The fifth control means is provided on at least one of the first transmission line and the second transmission line, and controls a group delay time difference between the optical signals of the two polarized wave components substantially perpendicular to each other by performing a predetermined processing for an optical signal transmitted on at least one of the first transmission line and the second transmission line.
The fifth control means includes at least one of the first, second and third still further control means. The first still further control means is provided in at least one of the first and second gratings, and controls a distribution of temperature of at least one of the first and second optical transmission lines in longitudinal directions of the first and second gratings. The second still further control means is provided in at least one of the first and second gratings, and controls a distribution of stress applied to at least one of the first and second optical transmission lines in longitudinal directions of the first and second gratings. The third still further control means is provided in at least one of the first and second gratings, and controls a distribution of electric field applied to at least one of the first and second optical transmission lines in longitudinal directions of the first and second gratings.